Thermal sensors can be used in a variety of applications to measure heat flux and change in temperature associated with the exchange of heat due to a physical process or activity. If applied in the appropriate manner, thermal sensors can be used to measure other quantities, for example the flow of liquids or gases or the thermal properties of materials. Sensing of mass flow and heat transfer using thermal sensors is for example employed in climate control applications for buildings, temperature control of furnace and oven applications and for monitoring of drying processes.
The term “thermal sensor” is used hereinafter to mean a sensor formed from substrate and having one or more elements disposed thereon for heating and/or sensing properties of a substance or material. A microbridge sensor, for example as detailed in U.S. Pat. No. 4,651,564 to Johnson et al., is an example of such a thermal sensor. The microbridge sensor includes a flow sensor chip which has a thin film bridge structure thermally insulated from the chip substrate. A pair of temperature sensing resistive elements are arranged on the upper surface of the bridge either side of a heater element such that, when the bridge is immersed in the flow stream, the flow of the liquid or gas medium cools the temperature sensing element on the upstream side and promotes heat conduction from the heater element to thereby heat the temperature sensing element on the downstream side. The temperature differential between the upstream and downstream sensing elements, which increases with increasing flow speed, is converted into an output voltage by incorporating the sensing elements in a Wheatstone bridge circuit such that the flow speed of the gas or liquid can be detected by correlating the output voltage with the flow speed. When there is no fluid flow, there is no temperature differential because the upstream and downstream sensing elements are at similar temperatures.
Since the microbridge structure is burst proof, microbridge sensors are suitable for measuring clean gases, with or without large pressure fluctuations. However, the open nature of the microbridge structure can result in condensates from vapor being retained in the microbridge structure leading to uncontrolled changes in thermal response making the sensor measurements susceptible to error and instability. Furthermore, wires bonded to the heater and sensing elements retain particles suspended in the fluid and increase turbulence shifting flow response. Also, the wires are prone to damage in a high mass flux environment and during cleaning of the sensor.
Another example of a thermal sensor is a membrane-based sensor. Membrane sensors do not have an opening exposed to the fluid so that the liquid cannot enter the underlying structure. The membrane sensor enables accurate measurements to be made in harsh environments (condensing vapors, suspended particles, etc.). However, the membrane sensor includes an air gap sealed from the fluid by the membrane making the sensor susceptible to failure in high pressure environments due to deformation or bursting of the membrane. Also, the heating/sensing elements are typically wire bonded to other components and exposed to the fluid flow resulting in possible damage due to high mass flow or cleaning as in the case of the microbridge structure.
Another example of a thermal sensor is a microstructure thermal flow sensor having a microsensor die with a Microbrick™ or microfill structure which sensor is more suited to measuring fluid flow and properties under harsh environmental conditions. The microstructure flow sensor uses a Microbrick™ or microfill forming a substantially solid structure beneath the heating/sensing elements and has a passivation layer isolating the heating/sensing elements from the fluid so that the sensor is less susceptible to the effects of the fluid. Furthermore, the sensor uses through-the-wafer electrical connections instead of wire bonding overcoming problems associated with wire bonding. Although this type of microstructure sensor is capable of reliable, stable and rapid-response operation under harsh environments, the sensor still has limitations for the type of fluid flow environment in which the sensor can be used. Also, the microstructure can perturb the flow stream characteristics within the fluid so that the sensor cannot be used effectively.
The aforementioned problems demonstrate that there is a need to provide a thermal sensor system which is capable of accurately and reliably measuring properties of a fluid whilst achieving good chemical isolation from and minimal perturbation of the fluid.